Modern restorative dental procedures often require accurate color matching, such as for filling materials and for the fabrication of restorations such as crowns, implants, fixed partial dentures, and veneers. The materials used for these procedures, such as ceramics and other materials, can be skillfully formed and treated to closely match the shape, texture, color and translucency of natural teeth.
A widely used technique for determining and communicating tooth color information is a process referred to as “shade matching” whereby the dentist or technician visually matches a patient's tooth to one of a number of reference shade samples or shade tabs within one or more sets of standardized shade guides. The practitioner who performs the match records the identification of the matching shade tab and conveys that information to the dental laboratory where the restoration or prosthesis is then fabricated. The laboratory then uses its own set of the same shade guides to perform visual color evaluations of the restoration or prosthesis throughout the fabrication process.
The visual shade matching process can be highly subjective and subject to a number of problems. The initial matching procedure is often difficult and tedious, and it is not unusual for the process to take twenty minutes or longer. In many cases, there is no shade tab that perfectly matches the patient's teeth.
The problem of accurately modeling the color of a tooth is more complex than obtaining a close color match using shade tabs. The inherent shortcomings and limitations of both instrument-based and visual-based shade-matching systems can be more fully appreciated by considering the difficulties involved in matching the appearance of human teeth. Tooth color itself results from a relatively complex interaction of reflection, transmission, refraction, fluorescence, and scattering by a variety of organic and inorganic components. It is influenced by variations in tooth pulp volume, dentin condition, enamel composition, and other variations in the composition, structure, and thickness of the dental tissues. One result of this complexity is that color appearance and color measurement are greatly influenced by lighting geometry, surrounding colors, and other environmental factors.
As a further complication, color within a single tooth is generally not uniform. Color non-uniformities can result from spatial variations in composition, structure, thickness, internal and external stains, surface texture, fissures, cracks, and degree of wetness. As a result, measurements taken over relatively large areas produce averaged values that may not be representative of a tooth's dominant color. In addition, natural color variations and non-uniformities make it unlikely that a given tooth can be matched exactly by any single shade tab. This means that a method for conveying the distribution of color within a tooth, not just its average color, is required. Further, tooth color is seldom uniform from tooth to tooth. Therefore, the ideal color of a restoration may not be in visual harmony with that of an adjacent tooth or of any other single tooth in a patient's mouth. Moreover, people generally are particular about the appearance of their teeth. Understandably, they are quite intolerant of restorations that appear inappropriate in color.
In cosmetic dentistry, the fabrication lab often requires additional information in order to more accurately map tooth color in addition to simple shade matching. In practice, the dentist or technician may provide a photograph in addition to a shade tab, so that the fabrication lab can adjust color characteristics over different portions of the tooth. This helps to provide a type of color mapping for subjective use, with information that relates to the shade tab and shows how colors in other portions of the tooth vary from that of the shade tab.
It is often difficult to decide which tab matches most closely (or, conversely, which has the least mismatch) and to provide accurate information on color variation over the tooth surface. Frequently, the practitioner determines that the patient's teeth are particularly difficult to match, requiring that the patient then go in person to the orthodontics laboratory that will be fabricating the restoration. There, trained laboratory personnel can perform the color match and color mapping. In many cases, the patient may even need to return to the dentist and laboratory two, three, or even more times as the color of the prosthesis is fine tuned by sequential additions of ceramics or other colored materials. In a high percentage of cases, estimated to be nearly 10% for some dental prostheses, the visual color matching procedure still fails and the prosthesis that has been fabricated is rejected for color or visual harmony by the dentist or by the patient.
Considering the relative difficulty of the color matching task, and the further complexity of color mapping, a high rate of failure is not at all surprising. Visual color evaluation of relatively small color differences is always difficult, and the conditions under which dental color evaluations must be made are likely to give rise to a number of complicating psychophysical effects such as local chromatic adaptation, local brightness adaptation, and lateral-brightness adaptation. Moreover, shade tabs provide at best a metameric (that is, non-spectral) match to real teeth; thus, the matching is illuminant-sensitive and subject to variability due to normal variations in human color vision, such as observer metamerism, for example.
In response to the need for improved color matching and color mapping in dental applications, a number of approaches have been attempted. Conventional solutions to this problem are of the following general types:                (i) RGB-based devices. With this approach, an image of the entire tooth is captured under white light illumination using a color sensor. Tristimulus values are calculated over areas of the tooth surface from RGB (Red, Green, Blue) values of the 3-color channels of sensor, making use of a color calibration transform. Color analysis by RGB-based devices relies heavily on the quality of the captured image and requires robust calibration and may require use of the same camera for color-matching of tooth and prosthetic device. This requirement can be due to calibration of the camera itself as well as to color preprocessing that is performed within the camera in order to provide the RGB data; this preprocessing can vary significantly from one camera to the next, even for cameras from the same manufacturer. Maintaining accuracy tends to be difficult and measurements are compromised due to metamerism, in which the color measured is highly dependent upon the illuminant. This is particularly troublesome since dental measurement and imaging are generally carried out under conditions that differ significantly from natural lighting conditions. Examples using RGB measurement and employing a corresponding color transform in this way include: U.S. Pat. No. 5,766,006 entitled “Tooth Shade Analyzer System and Methods” to Murljacic; U.S. Pat. No. 6,008,905 entitled “Method and Apparatus for Determining the Appearance of an Object” to Breton, et al.; and U.S. Pat. No. 7,064,830 entitled “Dental Color Imaging System” to Giorgianni et al.        (ii) Colorimetric devices. Devices of this type are engineered to directly measure color as perceived by the human eye. With this type of device, illuminating light or reflected light (under white light illumination) is filtered at the three wavelength bands that correspond to the spectral response characteristic or color matching functions of the eye, and measured reflected signals are directly translated into tristimulus values. As with RGB-based devices described in (i), measurements from this type of device also suffer from metamerism. Some examples using this approach include those disclosed in U.S. Pat. No. 5,383,020 entitled “Method and Apparatus for Determining the Color of a Translucent Object Such as a Tooth” to Vieillefosse that requires a spectrometer and U.S. Pat. No. 6,867,864 entitled “Optical Measurement Device and Related Process” to Overbeck et al.        (iii) Spectrophotometric devices. These devices employ spectral reflectance for obtaining color data. Illuminating or reflected light is spectrally scanned, and light reflected by the tooth is recorded, using a photosensor, as a function of wavelength. Visual color, that is, CIE (Commission Internationale de L'Éclairage or International Commission on Illumination) tristimulus color information, is then calculated from the measured spectral reflectance curve. Spectrophotometric devices are not subject to the same tendency to metamerism inherent to colorimetric and RGB-based devices and, potentially, yield more accurate color measurements. It is significant to note, however, that the spectrophotometer is not an imaging device. The spectrophotometer is an instrument that measures the spectral content of incoming light over a small area using a photosensor. Examples of tooth color measurement using spectrophotometric devices include U.S. Pat. No. 4,836,674 entitled “Method and Apparatus for Determining Color, in Particular of a Dental Prosthesis” and U.S. Pat. No. 6,038,024 entitled “Method and Apparatus for Determining the Color Stimulus Specification of an Object” to Berner.        
Although the data obtained using the spectrophotometric approach provides advantages for color matching over colorimetric and RGB approaches, including elimination of metamerism, this approach has been found difficult to implement in practice. The use of a light scanning component for measuring different spectral components, generally employing a grating or filter wheel, tends to make the spectrophotometric system fairly bulky and complex. This makes it difficult to measure teeth toward the back of the mouth, for example. Attempts to alleviate this problem have not shown great success. As one example, U.S. Pat. No. 5,745,229 entitled “Apparatus for Determining Optical Characteristics of an Object” to Jung et al. provides a compact spectrophotometric device employing optical fibers to channel reflected light to an array of sensors, each sensor using a different spectral filter. However, as is true of spectrophotometric devices in general (iii, above), this device measures only a small area of the tooth surface at a time. To obtain a color mapping of an entire tooth surface requires numerous separate measurements with this approach. The image capture process is time-consuming and does not provide consistent results. Color mappings can be inaccurate using such an approach, since there can be considerable sensitivity to illumination and image capture angles and probe orientation during the imaging process.
In general, conventional methods that employ color filters, either at the illuminant end or at the sensor end, can be less desirable because they are subject to the limitations of the filter itself.
Thus, there is a need for an improved measurement apparatus that provides dental shade matching and mapping in a procedure that is straightforward to execute, having a high degree of accuracy, but without high cost or complex components.